Sound recording and reproducing



L. ESPENSCHIED SOUND RECORDING AND 'REPRODUCING Original Filed July8, 1920 INVENTOR Patented Mar. 6, 1928.

UNITED STATES PATENT OFFICE.

LLOYD ESPFiNSCHIED, F KEW GARDENS, NEW YORK.

SOUND RECORDING AND REPRODUCING.

Application filed July 8, 1920, Seria1 No.'394,708. Renewed May 6, 1927.

, with the sound translating devlces used totranslate between the variations in air pressure characterizing sound waves and waves of some other type. In the phonograph art of today these translating devices function to convert between sound waves and waves ofmechanical vibration and are of the familiar diaphragm form. Such diaphragm devices, because of their inherent natural periods, tend to transmit most effectively those component frequencies which resonate with.

the diaphragm and thereby give rise to distortion of the sound waves.

In accordance with the present invention, the translating device is caused to convert between air waves and electric waves and is of an entirel non-diaphragm type having no appreciab enatural period. By thus including in the recording and reproducing processes the intermediate step of conversion of the waves into electric form-certain important advanta es accrue as elucidated below which enab e the distortion to be much reduced. More specifically, the translating device is of a thermal type employing the agency of heat as the connecting link between the air and the electric waves.

.A further feature of the invention consists in amplifying the electric waves to the strength required to operate the recording device or the reproducing device as the case may be. The amplification may be such as 40 to enable the recording and reproducing to be effected without the use of a horn. Furthermore, the amplification may be made so great in recording as-to permit a recording artist considerable latitude of movement and so great in reproducing as to enable the reproduction to be heard throughout a hall.

Referring now to the drawing forming part of this specification, Fig. 1 illustrates the use of the thermophone as atranslatingvided with an amplifier anda recorder; ig.

device in a recording system, including the 3 indicates the use of these same or similar means in the reproducing of-sound waves from a record.

Tieferring to Fig. 1, 1 is an air-wave recelving horn, preferably of large flare 1n order to minimize distortion, to include a number of sound sources or to permit of considerable latitude of movement of the sound source. The horn may be omitted in order to render the sound pick-up substantially non-directional and to minimize reflection effects and their attendant effect upon the sound quality. 2 is the sound-sensitive element, the thermophone. This consists of a very small gauge heat-resisting wire such as platinum. It may be formed by the Vollaston process which consists in drawing down ,to a small gauge a silver wire with platinum core, and of then dissolving oif the silver for a short length, by means of acid, leaving a fine platinum filament. This short section of very fine wire is of small thermal capacity and its resistance follows very rapidly fluctuations in its temperature. "Several filaments in parallel, as illustrated, may be used to increase the energy-carrying capacity of the translator. The thermal filament is kept heated, up to say just below a red heat, by the exciting battery 3. 1

A highratio transformer, termed an input transformer, relates the low-impedance circuit of the thermophone with the high impedance input circuit of a vacuumtube or thermionic or audion amplifier shown schematically as 5. This type of amplifier is now well-known in acoustics and communication engineering and not being in itself generic to the present invention is not described in detail. 6 is a second such amplifier operated in tandem with the first. There may be used as many amplifiers as are required for obtaining the electrical energy required to actuate the recording device employed. The last amplifier unit is connected throu h a step-down output transformer 7 with t e recording or scribing device 8.

- The recording means 8 isan electromagnetic instrument of the same general type as the familiar telephone receiver. It comprises a shell-9 which supports an annular cap 10 which in turn'securesthe diaphragm 11.

connects with. the

the magnetizing or polarizing battery 13. The important feature of this device for the purposes of the invention is that it be highly damped or that it be given a very short (high frequency) natural period, or both. The high damping may be secured by padded members bearing against the diaphragm; or by air-damping by confining by means of a collar 14, a shallow chamber of air behind the diaphragm. The diaphra m is connected with any usual type of sty us arm 15 which may be pivoted, as shown, to cut lateral-wave records.

The operation of the recording system of Fig. 1 is as follows: Sound waves incident upon the heat-sensitive wire 2 vary the resistance of the same and cause current undulations of corresponding frequency in the primary winding of transformer 4. These variations cause alternating potential fluctuations across the secondary winding and between the grid and filament of vacuum tube amplifier 5. The alternations are reproduced in amplified form in the platelament output circuit. They are further amplified in like manner in amplifier 6 and pass to the electro-mechanical translating device 8. The diaphragm 11 follows the current variations in the electromagnet and causes the rocking arm 15 to vibrate in corresponding frequency, thus causing the stylus 16 to record undulations in the record 17. It will be understood that the recording device 8 is so mounted in the usual way as to give the crossover feed necessary to carve the record in a spiral groove.

The recording system of Fig. 2 is generally similar to that of Fig. 1 and differs therefrom in respect to the translating device employed in receiving the sound waves and in respect to certain details of the electromagnetic recording instrument. Sound 'waves entering the born 1' impinge upon the are 2 and by virtue of the variations in air pressure characterizing sound waves vary the effective resistance of the arc. The arc'is maintained by a direct current source 3' fed through choke coil 20. The fluctuations of the arc resistance in accordance with the sound waves cause corresponding variations in the potential acting across the condenser 21 and rimary of transformer 4'. These alternating potentials and resulting currents are amplified by amplifiers 5' and 6', and operate the recording device 8. This device differs from the corresponding one 8 of Fig. 1 in that the diaphragm 11, is made of parchment stretched over an annular support 19. The object of this construction is to impart 9. hi h natural frequency to the diaphragm to ring this frequency above the acoustic ran e or ordinary sounds, and thereby obviate istortion arising from the natural period bein within the working frequency range. iaphragm 11 is pm vided with a ferric armature 18 made as thin and light weight as possible. The vibrations imparted to the diaphragm system by the electromagnet 12 are transmitted to stylus 16 through arm 15 and are traced in the record 17 in the usual way.

In the recording devices 8 and 8 are represented two ways of obviating distortion in such electromagnetic devices, one by means of high damping as in the case of 8, and the other high natural frequency as in the case of 8'. It will be understood that these translating devices whose function it is to translate between electric and mechanical wave motion are not limited to the electromagnetic type, but may be of other types such as the electrostatic in which case a condenser plate is controlled by the charge impressed upon it and effects the motion of the stylus. Furthermore the record may be of the telegraphone type and the translator the magnetizing electro-magnetic. In the case of recording photographicall by controlling a beam of light in accor ance with the sound waves, the recordin translator may be a source of'light, the brilliancy of which is controlled'by the output current of the amplifier. For recording by the light method, the same circuit as is employed in Fig. 2 for air-electric translation may be duplicated on the output side of amplifier 6, in lieu of the electromagnetic translator 8. The light source may be an arc li ht similar to 2' with its two circuits, the irect current supply and the sound-frequency circuit, represented by the condenser 21 and primary winding of transformer 4;. In this case the li lit-intensity of the are is modulated by the sound-wave currents and the recording is effected without the use of any diaphragm device whatsoever.

The steps involved in the recording systems above described are those of (1) airelectric translation by non-diaphragm translators, (2) amplification and (3) translation onto the record. This same sequence of functions may be operated in reverse manner for the purpose of re roducing a record. This fact is indicate in Fig. 3 where the amplifiers 6 and 6", corresponding to those of the previous fi es, are shown reversed in their position in the cir-' cuit. Both of the two types of translating 7 devices of Figs. 1 and'2 are reversible in their action and need not be redrawn in Fig. 3. The operation in reproducing a record is that the record, upon being played, generates electric currents in the electromagnetic translating device (8 or 8') these currents are amphfied and the amplified currents actuate the thermal electric-air translators (2 or 2' and create sound waves. The variations in t e heat developed by the thermal element (of either 2 or 2) cause variations in the immediately surrounding of a diaphragm as in the ordinary telephonetransmitter.

2. The record-taking or reproducing means which is rendered substantially distortionless and which may be mechanical, magnetic, light or other type depending upon the form of record employed.

3. The amplifier whereby the waves are amplified to the desired volume and which overcomes the reduced efiiciency of the high quality translating devices.

A It will be appreciated that many variations are possib e in the arrangements and devices employed for recording and re roducing sound waves, without departing rom the spirit and scope of the invention as defined .in the claims. a

What I claim as new and desire to secure by Letters Patent is 1. In a phonograph system for translating sound waves without sensible distortion, the combination of an electric transmission circuit, a device .for translating sound waves between the circuit and the record, said device comprising an operatmember havin a natural period of vi ration substantia y higher than the frequencies of vibration to be translated there- Ey, and a substantifall non-distorting nonaphngm yp 0 between the sound waves in the air and the electric circuit.

evice for translating 2. In a phonographsystem for translating.

sound waves without-sensible distortion, the combinatlon of an electric transm1ss1on c1rcult, a device for translating sound waves between the circuit and the record, said device comprising an operating member having a natural period of vibration substantially higher than the frequencies of vibration to be translated thereby, a substantially non-distorting non-diaphragm type of device for translating between the sound waves in the air and the electric circuit, and a substantially distortionless electronic type of amplifier included in said electric circuit.

3. In a phonograph system for translating sound waves without sensible distortion, the combination of an electric circuit, means for converting air waves into electric waves in said circuit, said means comprising a translating ,device, the electric re-- sistance of ,which is controlled directly by the air waves without the intervention of a vibratin member, means in said circuit for ampli ing said waves, and an electroma netlc evice forrecording said waves, said device i-being substantially equally responsive to all frequencies in the range to be recorded thereby. 4. Ina phonographsystem for translating sound waves without sensible distortion, the combination of an electric circuit, means for converting air waves into electric Waves in-said circuit, said means comprising a resistance element sensitive directly to sound waves impinging thereon, and including a source of electric current, means in said circuit for amplifying said waves, and an electromagnetic device for recording said waves, said device being substantially equally responsive to all frequencies in the range to be recorded thereby.

In testimony whereof, I have si ed my name to this specification this sixt day of 'July, 1920.

LLOYD ESPENSCHIED. 

